Problem using Voronoi texture as Cycles displacement (2.8)
Displacement is usually done along faces normals. This is the role of the 'displacement vector' to help to do that.

If you use any vector like a color as displacement in the output node, the displacement will operate directly in the directions given by this vector and this will not consider faces orientations. This is the case in the image in your question.

So simply insert a 'displacement vector' (which can be found in the 'vector' category) between your displacement calculation and the output.

Here is a view with on left an exaggerated offset when 'displacement' is not used. And on the right, the same with a displacement vector.

(Note that the 'screen' node is not useful here I think).



Blender file for 2.8, Cycles experimental using adaptive subdivision: